1. Field of the Invention
The present invention concerns a method to determine a kidney function parameter of kidneys of an examination subject by magnetic resonance tomography, and a magnetic resonance system for this purpose.
2. Description of the Prior Art
The kidney is an organ that serves to eliminate end products of metabolism. The kidney filters these substances (what are known as urophanic substances) from the blood stream and forms urine which, after intermediate storage in the urinary bladder, is excreted from the body via the urinary tract. Primary urine is filtered from the blood, and valuable plasma components (for example sugar, amino acids and electrolytes) are resorbed into the blood stream. Moreover, a majority of the water filtered out is resorbed, wherein the primary urine is concentrated into secondary (final) urine, which ultimately is conducted via the ureter to the urinary bladder. Parameters that characterize the function of the kidney are, for example, the concentrations of the urophanic substances in the blood or the renal clearance, for example the inulin clearance or the creatinine clearance. Since creatine or inulin are essentially neither secreted nor resorbed, the glomerular filtration rate of the kidneys can be determined from the renal clearance of these substances. Essentially, this indicates the plasma volume from which the urophanic substance has been removed per time unit. The kidney function can thus be characterized with the use of physical quantities.
The determination of these quantities is important for various applications. A determination of the parameters cited above can be useful for scientific studies and for the generation of statistics, for example. Furthermore, databases can be developed that associate a certain glomerular filtration rate with an age or a weight of a person, for example.
Such parameters also can be used in order to establish whether a kidney is operating in its normal parameter range.
Conventional methods to characterize the kidney function use, for example, a determination of the creatinine level in the blood. However, this is very time-consuming since blood must first be extracted from a patient and sent to a laboratory for analysis. The result of the blood test is then generally available only on the next day. Furthermore, the inaccuracy of such a creatinine measurement is disadvantageous. This primarily is due to the fact that the correlation between creatinine level in the blood and the glomerular filtration rate of the kidneys is only indirect across large ranges, such that only a very imprecise determination of the filtration rate is possible. For example, a variation of the glomerular filtration rate only has an effect in the blood as of a change on the order of 50% of the creatinine value.
A further known method uses a Gd-containing contrast agent in order to characterize the kidney function of a patient. The contrast agent is administered to a patient and the excretion of the contrast agent is measured. However, if the patient suffers from a kidney insufficiency, the administration of such a contrast agent is dangerous, in particular since the risk thereby increases of developing a nephrogenic systemic fibrosis. With the establishment of a connection between the administration of contrast agents and the occurrence of an NSF, by now some contrast agents may no longer be administered to patients with limited kidney function.
Consequently, it is desirable to characterize the function of kidneys of an examination person without having to administer an incompatible contrast agent. In particular, it is desirable to conduct a precise characterize in a relatively short time period.